1. Technical Field
This disclosure relates to a voltage rising/falling type switching regulator and its operation control method in which voltage rising/falling operations are switched automatically, and more particularly to operation of a switching regulator at a time of switching voltage rising operation and voltage falling operation.
2. Description of the Related Art
FIG. 6 shows a voltage rising/falling type switching regulator according to the related art (for example, Japanese Laid-Open Utility Model Application No. 6-70486). FIG. 7 shows the relationship between input voltage Vi and output voltage Vo in the voltage rising/falling type switching regulator of FIG. 6.
In the voltage rising/falling type switching regulator of FIG. 6, at a time of voltage falling operation, a voltage falling control circuit 101 controls the on-duty cycle of a pulse inputted to the base of a switching transistor Q101 for voltage falling, so that a division voltage Vs is set to a reference voltage (Vref1+Vref2). And at a time of voltage rising operation, a voltage rising control circuit 102 controls the on-duty cycle of a pulse inputted to the base of a switching transistor Q102 for voltage rising, so that the division voltage Vs is set to a reference voltage Vref2.
As shown in FIG. 7, in the region where the input voltage Vi is relatively small and the voltage rising operation is performed, the output voltage Vo is set to a voltage Vo1, and in the region where the input voltage Vi is relatively large and the voltage falling operation is performed, the output voltage Vo is set to a voltage Vo2 larger than the voltage Vo1. On the other hand, when the input voltage Vi falls within a region where the output voltage Vo is in a range between the voltage Vo1 and the voltage Vo2, the non-control state in which neither the voltage rising operation nor the voltage falling operation is performed takes place.
There is known an operation control method of a switching regulator according to the related art (for example, Japanese Laid-Open Patent Application No. 2005-57954). According to this operation control method, only one of an output of a PWM comparator for voltage falling control and an output of a PWM comparator for voltage rising control is used so that voltage rising operation and voltage falling operation may not occur simultaneously.
Moreover, according to the above method, the resistance for shifting the level of an output voltage of an error amplifying circuit is finely adjusted to avoid occurrence of the non-control state, and the setting is performed so that switching between voltage rising operation and voltage falling operation is performed instantaneously.
There is known another operation control method of a switching regulator according to the related art (for example, Japanese Laid-Open Patent Application No. 2003-125576). According to this method, a switching frequency is made large for a region where the input voltage and the output voltage are near to each other, and the difference in the power loss between at the time of shifting from voltage rising operation to the non-control state and at the time of shifting from the non-control state to voltage falling operation is made small, so as to attain smooth operation shifting.
However, the switching regulator of Japanese Laid-Open Utility Model Application No. 6-70486 has a problem in that when the switching regulator is in the non-control state, both the switching transistor for voltage falling and the switching transistor for voltage rising stop operation, and the power conversion efficiency improves. However, at the time of shifting from voltage falling operation or voltage rising operation to the non-control state, or at the time of shifting from the non-control state to voltage falling operation or voltage rising operation, the power loss changes abruptly and the output voltage Vo fluctuates sharply. Moreover, in order to provide the non-control state, it is necessary to carry out the setting so that the output voltage value Vo1 at the time of voltage rising operation is smaller than the output voltage value Vo2 at the time of voltage falling operation. Whenever switching between voltage rising operation and voltage falling operation is performed, the sharp fluctuation of the output voltage will occur.
The operation control method of Japanese Laid-Open Patent Application No. 2005-57954 requires precise adjustment of the resistance in order to shift the level of the output voltage of the error amplifying circuit. If the adjustment of the resistance deviates, the non-control state will occur. To avoid this, it is necessary that temperature characteristics of the adjusted resistor are in conformity with temperature characteristics of the circuit which sets up the level of a triangular wave oscillating circuit, to raise the adjustment accuracy. Thus, there is a problem that the cost for the adjustment of the resistance increases.
In the operation control method of Japanese Laid-Open Patent Application No. 2003-125576, the clock frequency of the converter changes according to the input voltage, and it is likely that noises occur over a wide range of frequencies. Thus, the operation control method is not appropriate for use in power supplies of wireless communication devices or the like.